Digital radio receivers include an analog front end that receives FM band signals and an analog to digital (A/D) converter that converts the FM band signals to digital FM band signals. A decoder decodes the digital FM band signals to generate digital audio signals. The decoded digital audio signals provide CD-like audio with improved sound quality over analog-based audio signals.
Digital radio receivers also preferably maintain backward compatibility with existing analog-based audio signals in the FM band. When the digital radio receivers operate in a hybrid mode, the FM band signals contain both analog audio signals and digital audio signals. In the hybrid mode, a conventional analog FM receiver demodulates the FM band signals to produce analog audio signals and the decoder decodes the FM band signals to produce the digital audio signals.
For example and referring now to FIG. 1A, automobiles 10 traveling down a road may demodulate and/or decode FM band signals from a first radio transmitter 12, a second radio transmitter 14, and/or a third radio transmitter 16. The first transmitter 12 generates FM band signals that contain modulated analog audio signals. The second transmitter 14 generates FM band signals containing modulated analog audio signals and encoded digital audio signals, respectively. The third transmitter 16 generates FM band signals containing encoded all-digital audio signals. Digital radio receivers 18 in the automobiles 10 must be able to tune and receive all of these different types of FM band signals. As can be appreciated, stationary FM receivers may also receive the different FM signals. When the digital radio receiver is tuning into a radio program, it may not know which system is used to encode the received signal, which may cause tuning delays that are undesirable.
When the digital radio receiver first tunes into a program channel, the digital receiver determines the type of signal that is transmitted so that an appropriate receiver decoding algorithm can be applied. Currently, digital signals are typically coded using a hybrid In-Band-On-Channel (IBOC) Frequency Modulated (FM) system, which typically transmits the digital information at a lower power level than the analog host FM signal.
Referring now to FIG. 1B, a typical power spectrum of the hybrid signal is shown. Fc denotes the center frequency of a program channel. Fc is located in the range of 88-108 MHz for FM broadcasts in the US. In the hybrid IBOC-FM mode, the digital signal portion is transmitted in sidebands on both sides of the analog host signal, in other words Fc−F1 and Fc+F1. For example, F1 may be equal to 200 kHz. In this example, the composite signal is 400 kHz wide while traditional FM program channels are 200 kHz wide.
The digital information is typically transmitted using an Orthogonal Frequency Division Modulation (OFDM) approach. The details of one suitable transmission scheme are described in [“HD Radio Air Interface: Design Description—Layer 1 FM”, Reference C, Mar. 7, 2003] (hereinafter “the Draft Standard”), which is available from the National Radio Systems Committee (NRSC) that oversees the standard setting activities for digital radio receivers and which is hereby incorporated by reference in its entirety.
As more consumers purchase digital radio receivers, traditional analog FM transmission will eventually be turned off and the entire frequency band will be used for transmission of digital radio that will enable higher quality audio and/or provide bandwidth for other data programming. This system configuration is the all digital IBOC mode, which is described in the Draft Standard of the IBOC-FM system identified above.
Referring now to FIG. 2, a functional block diagram of the physical layer of one suitable IBOC-FM transmitter 30 is shown. A detailed discussion of IBOC-FM transmitter 30 will not be provided but can be found in the Draft Standard. The digital data bitstream from Layer 2 40 is output to a scrambler 44 and then to a channel encoder 46. The scrambler 44 randomizes the data. The channel encoder 46 introduces redundancy to protect the bits from noise and channel fading. The coded frame of bits is output to an interleaver 48, which rearranges bits of one symbol or frame with bits of another symbol or frame. The output of the interleaver 48 is input to an OFDM subcarrier mapper 52, which maps pairs of bits to Quadrature Phase Shift Keying (QPSK) symbols before being assigned to OFDM subcarriers. An output of the OFDM subcarrier mapper 52 is input to an OFDM signal generator 54 and a transmission subsystem 58.
Depending on the mode of operation for the current frame of data, a different number of OFDM subcarriers may be used. In each OFDM symbol, some subcarriers are reserved for control information. These subcarriers are known as the Reference Subcarriers. The control signal is coded separately from the data frames. The control signal contains pilots for synchronization and information on the modes of operation of the current frames. After the subcarrier mapping, the transmitter performs an Inverse Discrete Fourier Transform (DFT) (that can be implemented using a Fast Fourier Transform (FFT)) and appends a cyclic prefix to form an OFDM symbol in the time domain. The OFDM symbol is then transmitted by the transmission subsystem 58.